The present invention relates to radar controlled weapons systems and, more particularly, to a method and apparatus for generating real-time high resolution Synthetic Aperture Radar (SAR) imagery from an airborne platform of a translating ship under the influence of roll, pitch, and yaw motions characteristic of sea state conditions.
A highly resolved ship image on an airborne display permits targeting to a particular part of the ship for the purpose of standoff command guidance weapon delivery. As such, this invention relates to airborne SAR systems used for generating real-time high resolution imagery of a ship target under the influence of sea state conditions and for accurately measuring and tracking the range and azimuth angle of a designated resolution cell within the aforesaid displayed target area so as to enable the accurate delivery of an air-to-ground missile or glide bomb from the SAR bearing aircraft to the ship target. The manner of weapon guidance depends upon reducing to zero the relative range and azimuth angle between weapon and designated target resolution cell.
Existing airborne synthetic aperture radar systems have the capability for producing a high resolution image of stationary ground targets by virtue of the rotational motion of the SAR with respect to the target field. High resolution in both range and cross-range (azimuth) directions are achieved with short-pulse radar techniques for range resolution, and by digital signal doppler processing for azimuth resolution, so as to produce a two-dimensional image. The application of such known SAR processing techniques to a ship at sea, however, can readily produce distorted and improperly focused ship imagery, precluding ship recognition, because of ship rotational motions brought about by sea state conditions.
A system for ameliorating some of the adverse effects brought about by ship rotational motions is described in a commonly assigned copending United States patent application of Sol Boles, Ser. No. 389,367, filed concurrently herewith, entitled "Range/Azimuth Ship Imaging For Ordnance Control", the disclosure of which is incorporated herein by reference. The disclosed invention provides an undistorted two-dimensional image of the ship from a direct plot of range versus interferometrically determined azimuth angle of all essential scatterers comprising the ship. Notwithstanding the advantages achieved in conjunction with the disclosed invention, it does have certain operational limitations and drawbacks in certain applications. For example, the aforementioned techniques may not be capable of generating SAR imagery of high quality when the ship target is located in a high sea state environment because of the resulting degradation in the signal to noise ratio.
The invention disclosed herein is related to that described in the aforementioned application of Boles, entitled "Range/Azimuth Ship Imaging For Ordnance Control", but by a unique processing implementation, is capable of achieving a significant advantage in ship image quality and signal to noise ratio. The latter advantage, in turn, permits operation to greater ranges than would otherwise be possible by the techniques described in "Range/Azimuth Ship Imaging For Ordnance Control" for given transmitter and antenna parameters. This advantage is brought about by processing and displaying a high resolution range/doppler image of the ship target, with the distortions inherent in such a presentation due to ship scatterer rotational motions brought about by sea state conditions removed in accordance with the teachings of this invention, rather than processing and displaying a range/interferometrically determined azimuth angle image.
As those skilled in the art will recognize, the range/doppler representation could have a very substantial signal to noise advantage over the range/azimuth approach to imagery because of the image quality limitations brought about by angle glint errors associated with the latter.
It is accordingly an object of the present invention to provide a method and apparatus for generating high resolution synthetic aperture radar displayed imagery from an airborne platform of a ship under the influence of sea state conditions.
It is another object of the present invention to provide a method and apparatus for determining the cross line-of-sight relative velocity associated with aircraft and ship rotational motions requisite to achieving prescribed cross-range (azimuth) resolution in the displayed image representation of the ship.
It is a still further object of the present invention to provide a method and apparatus for eliminating the distortions inherent in range/doppler ship imagery brought about by ship rotational motions accompanying sea state conditions by forming a scaled SAR high resolution range/doppler image projection of the ship on the basis of interferometric azimuth angle measurements derived from doppler processed range sampled data, so as to permit the cursoring and tracking of a particular range/doppler resolution cell of the ship target for the purpose of carrying out standoff command guidance weapon delivery to said target.